In processing wafers of semiconductor material for fabrication into finished semiconductor devices, numerous processing steps are required. These steps may include grinding, polishing, vapor deposition, formation of various other types of elements on the wafer, heating in ovens, aligning wafers to predetermined orientations for photolithography, cleaning, scribing and cutting, and numerous other operations. Since each of these operations is distinctly different from the others, the operations are performed at numerous different stations during the preparation of the wafer. Historically, such wafers were often processed in batches with the wafers being collected in containers or "boats" at the end of each processing step and then manually transported and inserted into the next processing station. In the continuing quest for increased efficiency in processing, it has become desirable to process the wafers in a manner and structure as nearly automated and continuously operating as possible. Accordingly, in modern wafer processing facilities numerous different processing stations have now been linked together by conveyor structures which transport the wafers more or less continuously through each processing station and from each processing station to the next. While fully continuous operation is most desirable, such is generally unobtainable due to varying processing rates at the various processing stations, such that certain stations can process wafers very rapidly while others may operate at a considerably slower throughput rate. Additionally, malfunctions or processing difficulties may cause occasional backups in the processing sequence or interruptions in the feeding of wafers.
In order to mitigate the effects at later processing stations of any such processing problems, it is important that some form of buffer storage apparatus be provided to accumulate wafers coming from one station when the next station is not yet available and for providing a source for a continuous supply of wafers when the incoming supply from the previous station is interrupted. For the purposes of quality control it is also necessary that some means be provided for maintaining the identity of different groups of wafers being processed, in order that quality control and reject rate evaluation may be identified to specific groups of wafers. Prior art of this nature, such as in IBM Technical Disclosure Bulletin, Vol. 18, No. 11, has taken the form of an elevator consisting of parallel belts having opposed wafer supporting bars for lifting and storing a predetermined number of wafers from one conveyor, with transfer mechanisms for them moving the wafers to another conveyor. Other prior art apparatus, such as U.S. Pat. No. 3,353,651, has provided a bay of fixed dimensions off to the side of the conveyor for receiving any excess number of conveyed items moving along the conveyor. All of these prior art devices have suffered from one or more disadvantages. Among these disadvantages have been the inability to place certain of the prior art devices, such as those of U.S. Pat. Nos. 3,731,823 and 3,945,505, in line with a continuing conveyor, thus requiring a plurality of conveying paths and means for transferring wafers from one path to another. Another disadvantage frequently encountered, as with U.S. Pat. No. 3,122,229, has been the provision of a nonremovable storage chamber which can store only a limited number of incoming items, and the inability to maintain the identity of different groups of wafers being processed, which is necessary in order that quality control and reject rate evaluation may be identified as to specific wafer groups.